Using histocompatibility antigens of mice and men we will examine the differentiation and the structure of membranes. Experiments will be made to analyze the fluidity of the membrane and the ways in which protein mobility is restricted in the cell surface. Serological analyses, using fluorescent antibodies for direct observation and for fluorescence energy transfer measurements, will determine the spatial relationships between membrane components. Hormone binding studies will attempt to show the effect of surface antigens on association of hormones with the cell surface.